1. Field of the Invention
The present invention relates to an induction heating roller apparatus and to an image formation apparatus, which are provided with the fixing apparatus.
2. Description of the Prior Art
Heating rollers, which employ halogen lamps as heat sources, are used in the prior art to thermally fix a toner image. However, the halogen lamp heat sources are inefficient and require a large amount of power. Accordingly, a technique involving induction heating is being developed to solve such problems.
Japanese Laid-Open Patent Publication No. 2000-215974 describes an exciting coil, which is arranged near a heated object. The exciting coil generates an induction current in the heated object, which is a magnetic heating roller. The exciting coil is formed by winding a coil in a planar manner along a curved surface of the heated object. A magnetic core is arranged along the curved surface or the exciting coil on the side opposite to the heated object at the longitudinal ends of the exciting coil (first prior art example).
Japanese Laid-Open Patent publication No. 2000-215971 describes an induction heating apparatus having a heating rotor, or heating roller, which generates heat by means of electromagnetic induction, and a magnetic flux generating means, which is arranged in the heating rotor. The magnetic flux generating means includes a magnetic core and an electromagnetic conversion coil, which is wound about the core. The magnetic core includes a core portion, about which the electromagnetic conversion coil is wound, and a magnetic flux induction core portion. The magnetic flux induction core portion, which has a magnetic gap between its distal ends, concentrates magnetic flux at part of a heating rotor rather than the core portion (second prior art example).
The first and second prior art examples employ a heating technique that uses eddy current loss (hereafter referred to as eddy current loss technique). Such heating technique works under the same principle as that applied to IH jars. The frequencies of the high frequency employed in the eddy current loss technique is about 20 to 100 kHz.
In comparison, Japanese Laid-Open Patent Publication No. 59-33787 describes a high frequency induction heating roller. The high frequency induction heating roller includes a cylindrical roller body, or heating roller, which is formed by a conductive member, a cylindrical bobbin, which is arranged in the roller body in concentricity with the roller body, and an induction coil, which is spirally wound about the periphery of the bobbin. When current flows through induction coil, the induction coil, which induces induction current in the roller body, is heated (third prior art example).
In the third prior art example, the cylindrical roller body functions as a secondary coil, which is a closed circuit, and the induction coil functions as a primary coil. This causes transformer coupling between the primary and secondary coils and induces a secondary voltage in the secondary coil of the cylindrical roller body. Based on the secondary voltage, a secondary current flows in the closed circuit of the secondary coil. This is a heating technique (hereafter referred to as a transformer technique) that heats a secondary resistor, which heats the cylindrical roller body. The transformer technique, which has a high stationary efficiency since its magnetic coupling is stronger than the eddy current loss technique, entirely heats the heating roller. Thus, the transformer technique is advantageous in that is simplifies the structure of a fixing apparatus in comparison to the first and second prior art examples. Further, when the operational frequency is 100 kHz or greater, and preferably a high frequency of 1 MHz or greater, the Q of the induction coil may he increased to increase the power transmission efficiency. This increases the total heating efficiency and reduces power consumption. Further, the heat capacity is much smaller than that of the eddy current loss technique. Accordingly, the transformer technique is preferable for increasing the speed of thermal fixing.
The inventors have invented a transformer coupling technique that efficiently heats the heating roller. In the transformer coupling technique, by forming a closed circuit, the secondary reactance of which is substantially equal to a secondary resistance of the heating roller that is air-core transformer coupled to an induction coil, the efficiency for transmitting power from the induction coil to the heating roller increases. This efficiently heats the heating roller. An application for a patent for this invention was applied for in Japanese Patent Application No. 2001-016335. The invention reduces power consumption for induction heating of the heating roller and facilitates increasing the speed of thermal fixing.
In an image formation means, such as a copy machine or a printer, paper on which images are formed is selected from multiple sizes. To cope with such function, the heating area of the heating roller must be changed in accordance with the paper size.
As for the trans-method, it is possible to render the heating area of the heating roller changeable in the axial direction by placing a plurality of the induction coils in a dispersed state in the axial direction of the heating roller and selectively driving the induction coils as a suitable configuration of the induction coils for the heating roller. It is thereby possible to meet the requirement and heat only a necessary area so as to avoid wasteful power consumption.
In the case of the fixing apparatus using the heating roller, however, it is necessary, for the sake of fusion-binding toner on paper, to manage it so as to implement even temperature distribution in which temperature anomaly of the heating roller is within plus or minus 15 degrees C.
Thus, as shown in FIG. 24A, if a plurality of induction coils 102 are placed with spacing among themselves inside a heating roller 101 and are energized by a high frequency source (not shown), the induction current runs in the closed circuit of the secondary coil of the heating roller 101 which is then heated. And the heating roller 101 at this time shows a temperature distribution characteristic as shown in FIG. 24B. As for FIG. 24B, the horizontal axis shows a position of the heating roller and the vertical axis shows the temperature respectively. As shown in the drawing, the point indicated by a symbol a in the drawing at which the induction coil is placed shows a high temperature, whereas the point equivalent to the spacing between the induction coils is at a temperature b which is lower than the high temperature a. An induction heating coil apparatus having such temperature distribution shows the temperature distribution characteristic far better than that of the induction heating coil apparatus in the past. However, there remains a room for further improvement in order to implement the induction heating coil apparatus having the even temperature distribution which is satisfactory.
In order to solve the temperature anomaly of the heating roller 101, it is thinkable to place the induction coils 102 in the proximity. However, the traders generally think that, if the induction coils 102 are placed in the proximity, a considerable portion of the flux generated from the induction coils 102 interlinks with the adjacent induction coils 103 so that, as the power transmission efficiency from the induction coil to the heating roller becomes lower, a high power transmission efficiency cannot be obtained. Therefore, when adjacently placing the plurality of induction coils 102 which are individually energized, they are not placed in sufficient proximity.
For the above reason, appearance of the induction heating roller apparatus having solved the problem of the evenness of the temperature distribution in the heating roller 101 is eagerly desired.
Nevertheless, in the case of placing the plurality of induction coils in a dispersed state in the axial direction of the heating roller, there are the following problems in addition to the above-mentioned problem of the evenness of the temperature distribution. To be more specific, there arises a significant potential difference between the adjacent induction coils so that it is necessary to provide a predetermined insulation distance between a pair of the adjacent induction coils according to the potential differences between them. This problem of the electric insulation distance is also a reason that it is difficult to shorten the spacing between the induction coils.
As the induction coils and the high frequency power supply are generally placed at positions with spacing among them, they are connected via electric supply lines. Therefore, it is necessary to adequately perform insulation not only between the induction coils but also mutually between the electric supply lines and between the electric supply line and the induction coil.
Furthermore, to selectively drive the plurality of induction coils, it is necessary to connect each of the induction coils to the high frequency power supply independently of one another. For that reason, it becomes difficult to secure the electric insulation distance among a plurality of the electric supply lines extended from each high frequency power supply.
To summarize the above, each of the requirements described above must be satisfied in order to evenly heat the heating roller in its axial direction and render the heating area switchable. However, it was difficult for the related arts in the past to meet all the requirements.